A summit meeting between President Reagan and Soviet leader Mikhail Gorbachev in the next few months appeared unlikely yesterday after an announcement that Gorbachev will not attend the opening of the U.N. General Assembly this fall.

"It's our understanding that Gorbachev's not coming to the U.N. this fall," deputy White House press secretary Robert Sims said. "It's what the Soviets have been telling us for the past two or three weeks."

Sims was responding to a news report quoting an unidentified U.S. official in Moscow as saying Gorbachev would not attend the U.N. session. The official did not rule out the possibility of a summit meeting later this year.

Soon after Gorbachev took over the Soviet governmental reins on March 11, Reagan invited him to Washington. In a return letter, Gorbachev expressed interest but did not specify a date or place, and the matter is now being discussed in diplomatic channels.

Reagan said at his news conference Tuesday that "I have to be optimistic" about a prospective get-together with Gorbachev and referred to discussions on the matter, adding, "But I can't give you any report on where those negotiations have taken us."